Immune-mediated autoimmune and inflammatory diseases are a major public health issue. Defining the regulatory mechanisms that normally function to prevent pulmonary inflammation is therefore key to understanding the etiology of these diseases, and for developing therapeutic strategies to boost these activities in patients. Regulatory T cells (TR) expressing the transcription factor Foxp3 play a critical role in preventing autoimmunity and limiting immune-mediated inflammation. We have shown that during type-1 inflammatory responses, Foxp3+ TR upregulate the Th1-specifying transcription factor Tbx21 (T-bet), and that T-bet expression is critical for proper TR homeostasis and function during Th1-mediated inflammation. Therefore, the goals of this proposal are to determine in detail how loss of T-bet specifically within Foxp3+ TR impacts the initiation, progression and termination of Th1 responses in vivo (Specific Aim 1); analyze at the molecular level how Foxp3 and T-bet combine to control the expression of genes involved in Th1/TR differentiation, homeostasis and function (Specific Aim 2); and to identify the cytokines and cellular signals that control the phenotypic and functional differentiation of different TR subsets (Specific Aim 3).